(83c) Ultrasonic Reflectometry for Membrane Applications: Current Work and Future Opportunities
This presentation will provide an overview of the adaptation of ultrasonic reflectometry (UR) for the non-invasive, real-time characterization of membranes and membrane processes with fine-scale spatial and temporal resolution. UR is a relatively inexpensive technology that can be readily installed on commercial membrane modules for a wide range of separations applications. UR analysis is based on the principle that a sound wave is affected by the media through which it travels. Thus, changes in the transit time, attenuation, scattering and frequency content of an acoustic waveform can provide information on the properties of the media. Ultrasonic waves are both transmitted and received using piezoelectric transducers that are acoustically coupled with the object of interest. The resulting waveform is characterized by the magnitude and phase of the spectrum of the received signal. Under appropriate conditions, this waveform is reflected and transmitted at each interface encountered within the material being analyzed. Implementation of the UR technique for membranes performing a separation involves appropriately mounting one or more transducers on the membrane module; the transducers are then connected to appropriate receiving and processing electronics. Since its proposed adaptation for membrane applications by MAST Center membrane researchers at the University of Colorado some 10 years ago, ultrasonic reflectometry (UR) has been successfully employed to study a number of imporant membrane phenomena including membrane formation, membrane compaction and membrane fouling. Most recently, UTDR has been applied to the characterization of membrane structure and defect identification. While the presentation will focus on the use of UR for membrane fouling applications, significant developments in these other areas of application areas will be highlighted. Of all the potential applications of UR, membrane fouling is perhaps the most important since the adverse effects of membrane fouling on performance represent the most critical limitation of membrane-based liquid separations. Adaptation and development of UR for characterizing membrane fouling has used pulse-echo mode, whereby the transducer alternates rapidly between transmitting an ultrasonic waveform and receiving its reflection from the various interfaces in the membrane module. Information can be obtained from the arrival time, amplitude and phase of the reflected wave. The condition and properties of a membrane module can be inferred from its acoustic signature, which is determined from the amplitudes and arrival times for all the relevant peaks in the reflected UR waveform. The acoustic signature provides a convenient metric for use in signal-analysis protocols. To date, UR has been successfully applied to flat-sheet, spiral-wound and hollow-fiber geometries. Rapid advances continue to be made in both the hardware and software technology that is used in UTDR. Transducers are now available with a wide range of focal lengths and frequencies ranging from KHz to GHz. Moreover, materials and techniques are becoming available that permit using these acoustic transducers in hostile environments. Lower cost and faster digital technologies are being developed that permit significantly improved data acquisition and signal analysis. Improved signal-analysis software lends itself well for use in microprocessors that will permit deconvolution and filter algorithms to be implemented in a manner that allows real time analysis of the complex UR waveforms. We believe that rapid advances in the supporting technology for UR will enhance its utility for both laboratory-scale studies and commercial-scale applications. This presentation will address necessary future developments that will make it possible for the membrane industry to capitalize on the power of UR technology to improve membrane products and processes.